1. Field of the Invention
This invention is directed to methods for extending the cycle life of solid, secondary electrolytic cells.
2. State of the Art
Electrolytic cells comprising an anode, a cathode and a solid, solvent-containing electrolyte are known in the art and are usually referred to as "solid electrolytic cells". One preferred class of solid electrolytic cells are rechargeable (secondary) lithium cells which comprise a solid electrolyte interposed between an anode comprising lithium and a composite cathode which comprises materials suitable for recycling (recharging) the cell after discharge.
A solid, secondary battery typically comprises several solid, secondary electrolytic cells wherein the current from each of the cells is accumulated by a conventional current collector so that the total current generated by the battery is roughly the sum of the current generated from each of the individual electrolytic cells employed in the battery. Such an arrangement enhances the overall current produced by the solid, secondary battery to levels which render such batteries commercially viable.
However, one problem encountered with the use of solid, secondary electrolytic cells in such batteries is limited cycle life for the battery, i.e., the number of rechargings the battery can accept before the battery is no longer able to maintain acceptable levels of capacity. Specifically, the cycle life of the solid, secondary battery is related to the cycle lives of the individual electrolytic cells comprising the battery. In general, when one of the electrolytic cells in the battery ceases to maintain acceptable levels of capacity, the battery must drain more current from the remaining electrolytic cells so as to produce the same overall level of current from the battery which results in a reduction of the capacity of the remaining electrolytic cells in the battery. In turn, this results in a significant reduction in the cycle life of these cells and hence that of the battery.
In assessing the causes of such reduced cycle life, the inventor has unexpectedly discovered that reduced cycle life in secondary electrolytic cells containing a solid, solvent-containing electrolyte interposed between the anode and a cathode is believed to arise, in part, from electrolytic solution depletion in the electrolyte during cell operation. Without being limited to any theory, it is postulated that during cell operation, the composite cathode acts as a sink for the electrolytic solvent found in the solid electrolyte and that, during cell operation, there is a migration or redistribution of such solvent from the electrolyte to the cathode. It is further postulated that after repeating cycling of the secondary cell, the migration becomes sufficiently pronounced that the amount of solvent in the electrolyte is reduced to the point that the cell impedance is increased due to an increase in resistance in the electrolyte. In turn, the increase in cell impedance leads to a reduction in cycle life for the electrolytic cell.
This problem of redistribution of electrolytic solvent during cell operation is compounded by the fact that increased cell impedance and hence shortened cycle life arises even when the maximum amount of electrolytic solvent tolerated by the manufacturing process is employed in the process of preparing the composite cathode and the solid electrolyte. Specifically, during cell manufacture, one preferred method for composite cathode preparation is to extrude a cathode paste comprising a cathode material and electrolytic solvent onto a current collector substrate and then uniformly distributing this paste over the substrate by a comma bar. In order to maintain suitable consistency of the cathode paste to permit these extrusion and distribution processes, the maximum amount of electrolytic solvent which can be employed in paste is typically about 50 weight percent based on the total weight of the paste.
Likewise, the solid electrolyte is preferably prepared from an electrolyte solution comprising a solid matrix forming monomer, an alkali salt, and an electrolytic solvent. The solvent is employed for the purpose of solubilizing alkali salts during operation of the electrolytic cell and to act as a plasticizer in the solid electrolyte whereas the solid matrix forming monomer is employed to convert the electrolyte solution from a liquid to a homogeneous solid after curing. In order to convert the electrolytic solution to a solid phase after curing, the maximum amount of electrolytic solvent which can be employed in the solution is typically no more than about 80 weight percent based on the total weight of the solution. When the electrolytic solvent in the solution exceeds this amount, the solid matrix forming monomer in solution is diluted to the point where formation of a solid matrix upon curing is inhibited.
Thus lies the heart of the problem. That is, notwithstanding the use of the maximum amount of electrolytic solvent during manufacture of the composite cathode, the composite cathode apparently has a higher capacity for such solvent. Accordingly, during cell operation, this increased capacity for electrolytic solvent leads to a redistribution of the solvent from the electrolyte to the cathode. Likewise, notwithstanding the use of the maximum amount of electrolytic solvent during manufacture of the solid electrolyte, this redistribution of electrolytic solvent during cell operation will eventually result in such a reduction in solvent in the solid electrolyte that cell impedance increases and eventually the cycle life terminates.